Because of the widespread dissemination of antibiotic resistance among pneumococci, novel strategies based on broader understanding of its pathogenesis are required for the prevention of pneumococcal disease. Dr. Weiser's group has been among the first to note spontaneous, reversible phenotypic variations (phase variation) in the pneumococcus. The transparent phenotype is more adherent to cultured human type II lung epithelial cells, and is associated with the ability to colonize the mucosal surface of the nasopharynx in an animal model of pneumococcal carriage. At least one of the endothelial receptors for transparent organisms is the receptor for platelet activating factor (PAFr). Chromosomal loci contributing to the expression of variation in colony opacity have been isolated and their contribution confirmed by the introduction of defined mutations in the pneumococcal chromosome. These mutants will be used to identify the genes involved in expression of colony opacity and to determine the molecular mechanisms controlling phase variations in their phenotype. Phenotypically stable mutants will be used to determine the structural basis of differences in colonial morphology. For example, some preliminary evidence suggests that changes in the exposure of the unique phosphorylcholine determinant of pneumococcal teichoic acid may correlate with phenotypic variation and may facilitate binding to the PAF receptor. Lastly, these phenotypically stable mutants will be used to determine the contribution of each phenotype to the different stages of pneumococcal infection including adherence and clearance by phagocytosis.